The technical field of this invention is the implantation of ions into targets, such as silicon semiconductors, and, in particular, to improved methods for generating a silicon heterostructure having a discrete, continuous, buried dielectric layer which is substantially free of defects.
Ion implantation techniques are particularly useful in forming a class of buried layer devices known as silicon-on-insulator (SOI) devices. In these devices, a buried insulation layer is formed beneath a thin surface silicon film. These devices have a number of potential advantages over conventional silicon devices (e.g., higher speed performance, higher temperature performance, reduced junction capacitance, and increased radiation hardness).
In one known technique, known by the acronym SIMOX, a very thin (0.1 micron-0.3 micron) layer of monocrystalline silicon is separated from the bulk of the silicon wafer by implanting a high dose of oxygen ions (e.g., up to about 3.0.times.10.sup.18 oxygen ions/cm.sup.2) into the wafer to form a buried dielectric layer of silicon dioxide (SiO.sub.2 having a typical thickness ranging from about 0.1 micron to 0.5 micron). Assuming that a continuous amorphous dielectric layer can be formed, this technique of "separation by implanted oxygen" (SIMOX), provides a heterostructure in which a buried silicon dioxide layer serves as a highly effective insulator for surface layer electronic devices.
While SIMOX technology is proving to be one of the most promising of the SOI technologies, there are a number of problems still associated with the manufacturing of SIMOX materials, as practiced in the art. One particular problem is the high doses of oxygen ions that must be implanted to ensure that a continuous amorphous buried oxide region will be formed. Generally, ion implants on the order of 1.5.times.10.sup.18 ions/cm.sup.2 (the so-called "critical dose") are required to ensure formation of a continuous layer of silicon dioxide. Such high implant doses require undesirably long implantation protocols (four to six hours), which are expensive to perform, strain the implant apparatus, and increase the number of problems associated with ion implantation, such as static charge build up and/or introduction of heavy metal or carbon contaminants. In addition, the high implant dose can cause irrepairable damage to the silicon lattice overlying the dielectric layer, causing threading dislocations in the device. (A maximum defect density of about 10.sup.5 defects/cm.sup.2 is considered acceptable for device-grade silicon). Finally, high doses can yield undesirably thick dielectric layers. The thicker the dielectric layer, the thinner the overlying silicon layer becomes, increasing the possibility of charge build-up within this layer, and reducing the radiation hardness aspects of the device.
A number of different methods for producing thinner buried oxide layers have been tried, using low (subcritical) implant oxygen ion doses (see, for example, Stoemenos et al. (1986) vol. 48, Appl. Phys. Lett. pp. 1470-1472; and Hemment et al., (1989) vol. B9, Nuclear Instruments and Methods in Physics Research, pp. 210-214). However, the resulting dielectric layers are generally discontinuous, containing an unacceptable number of silicon inclusions or silicon "islands" within the layer and at the upper interface with the overlying silicon, substantially reducing the effectiveness of the dielectric layer.
U.S. Pat. No. 4,749,660 (Short et al., filed Nov. 26, 1986), discloses a method of manufacturing SIMOX devices having "substantially homogeneous, relatively thin buried silicon dioxide layers". The method comprises implanting a subcritical dose of oxygen ions, followed by at least one "randomizing implant" (e.g., of silicon ions), and a low temperature annealing protocol. The effectiveness of this method is unknown, as no experimental data is provided.
EPO 298,794 (Margail, J. et al, filed Jun. 13, 1988), discloses a method of manufacturing SIMOX heterostructures having sharp interfaces, using a multiple implant protocol. The method involves implanting a total dose of at least 1.5.times.10.sup.18 ions/cm.sup.2 in a series of multiple partial implants using a constant beam energy and subjecting the wafer to a high temperature annealing protocol between each partial implant. The total ion dose implanted is not reduced, but the intermediate annealing steps are thought to reduce the build up of threading dislocations in the overlying silicon.
There exists a need for a novel method of manufacturing SIMOX heterostructures having thin buried oxide layers with sharp interfaces and which are substantially free of silicon islands. It is therefore an object of this invention to provide a method of manufacturing such SIMOX materials requiring lower oxygen ion doses and shorter implant protocols. Another object of this invention is to provide a method of manufacturing such materials which is rapid and cost efficient. Other features and objects of this invention will be apparent from the description, figures and claims which follow.
As used herein, "overlying silicon body" is understood to mean that portion of the silicon wafer (substrate) lying over the buried oxide layer, and on which the semiconductor device is to be built. "bulk silicon region" is understood to mean that portion of the silicon wafer (substrate) lying below the buried oxide layer. "Nucleation sites" are understood to mean localized regions of damage to the silicon lattice. "Silicon islands" and "silicon inclusions" are understood to mean isolated pockets of silicon. "Upper interface" and "lower interface", respectively, refer to the boundaries separating the buried oxide layer from the overlying silicon body and the underlying bulk silicon region. "Substantially free of silicon islands" is understood to mean a buried oxide layer sufficiently depleted in isolated pockets of silicon such that the performance of the insulating layer or the overlying device is not affected.